A process of visualizing image information through a static image, such as an electrophotographic process, has been widely employed in various fields of art. In the electrophotographic process, a static image is developed on a photoreceptor through a charging step and an exposing step, and the static image is visualized through a transferring step and a fixing step.
In the electrophotographic process, a static image is formed on a photoreceptor through a charging step and an exposing step, which is developed with a developer, and a toner image formed on a fixing substrate through a transferring step is heated and melted in a fixing step, to be fixed on the fixing substrate. In the fixing step, not only the toner but also the fixing substrate, such as transfer paper, are heated to a necessary temperature by a fixing member, such as a fixing roll, and thus the toner is fixed on the fixing substrate. When the heating on the fixing substrate is insufficient, only the toner is melted by heating with the fixing member, and is adhered on the fixing member, to cause the so-called cold offset. When the heating is excessive, the viscosity of the toner becomes too low, and a part or the whole of the fixing layer is adhered on the side of the fixing member, to cause the so-called hot offset. Therefore, the proper fixing condition in that neither the cold offset nor the hot offset is caused by heating with the fixing member is necessarily ensured.
In the fixing step, the temperature of the surface of the fixing member tends to be lowered from the prescribed temperature having been set to the fixing member due to heat transfer from the heated fixing member to the fixing substrate and latent heat off evaporation of moisture contained in the fixing substrate. In general, the fixing member is re-heated corresponding to the difference to the prescribed temperature, so as to obtain the prescribed temperature. However, in the case of the continuous fixing, for example, the surface temperature of the fixing member is liable to be lowered because the heat emitted from the surface of the fixing member is larger than the heat supplied to the surface of the fixing member. After further continuing the fixing, the relationship between the heat supplying and the heat emission is reversed, and the heat supplied becomes larger than the heat emitted, to cause the so-called overshoot. In order to solve the problem, for example, a method may be employed in that the temperature of the surface of the fixing member is detected with a sensor to precisely control, but such measures increase the cost, and thus it is generally not preferred. Because the temperature of the surface of the fixing member always fluctuates even though the prescribed temperature is constant, the fixing region is preferably as large as possible to obtain a stable image. This tendency is becoming remarkable in miniaturization and high-speed operation in recent years.
Along with the increase in demand of energy saving, the energy required in the fixing step, which consumes a large proportion of the total electric power consumption of a duplicator, should be decreased. In order to realize such energy saving and enhancement of the fixing region, it is necessary to further lower the fixing temperature of the toner. The decrease in fixing temperature of the toner realizes not only the energy saving and the enhancement of the fixing region, but also shortening of the warming-up time, i.e., the time required for the surface of the fixing roll to have the prescribed temperature, and prolongation of the service life of the fixing roll.
However, when the fixing temperature of the toner is decreased, it brings about decrease in glass transition temperature of the toner particles at the same time, and it is difficult to ensure both the low fixing temperature and the storage property of the toner. In order to ensure both the low fixing temperature and the storage property of the toner, it is important to maintain the so-called sharp melting property in that the viscosity of the toner is quickly decreased at a high temperature region while maintaining the high glass transition temperature of the toner. However, because the surface temperature of the fixing member always fluctuates due to fixing as described in the foregoing, the fixing performance fluctuates due to the setting of the temperature at which the sharp melting occurs. Particularly, in the case using a color toner, the gloss and the color mixing property of the surface of the fixed image are important. The fluctuation of the surface temperature of the fixing member greatly influences the gloss and the color mixing property, to remarkably change the gloss and the color mixing property of the fixed image from the initial stage to the later stage of the continuous fixing, and thus the reliability of the image quality is deteriorated.
Furthermore, when the fixing substrate has surface unevenness, there causes difference in heat supplying from the fixing member to the fixing substrate between the concave part and the convex part. In general, because the heat is easily transferred at the concave part in comparison to the convex part, the concave part is substantially fixed at a higher temperature than the convex part. As a result, the gloss changes due to the difference in temperature to deteriorate the reliability of the image quality.
The problem can be solved, for example, by increasing the molecular weight of the toner. This is a method in that the difference in gloss is decreased by extremely lowering the gloss itself. However, in the case where the color toner is fixed on a transparent film, the transparency cannot be obtained by this method.
It is also possible that the difference in fixing temperature is decreased by increasing the contact time of the fixing member and the fixing substrate, to lower the difference in gloss. However, this method involves a problem in that it cannot cope with the high-speed operation of a duplicator.
In order to solve the problems, JP-Laid open No. H5-341564 proposes a method, in which a high temperature fixing region is obtained by adding a gel component to a toner. However, the addition of the gel component lowers the transparency, to deteriorate the transparency on fixing on a transparent film, and thus the transmitted light through the film exhibits turbid colors. Therefore, this method cannot be applied to a color toner.
JP-Laid open No. H7-199583 and JP-Laid open No. H6-19204 propose that only a black toner has a low gloss to balance the gloss of a full color image. However, this method cannot cope with the difference in gloss on continuous duplication.
Furthermore, JP-Laid open No. H2-245775 proposes a method, in which a low molecular weight substance and an adhesive resin are added to a binder resin of a toner, and the contact time of the fixing member and the fixing substrate is determined, so as to obtain the desired gloss. According to this method, a toner exhibiting a wide fixing region and having the desired gloss and transparency can be obtained, but cannot cope with the difference in gloss on continuous duplication. Therefore, there is a high demand of a toner having good balance among gloss, transparency and fixing region.